The invention concerns an NMR apparatus with a superconducting magnet coil system, in particular, an NMR spectrometer, having a cryostat comprising an outer shell and a helium tank which contains the magnet coil system, and an NMR probe head which is disposed in a room temperature bore of the cryostat, which contains a cooled RF resonator for receiving NMR signals from a sample to be investigated, and which is cooled, together with the NMR probe head, by a cold head of a common, multi-stage, compressor-operated refrigerator.
A device of this type is disclosed in WO 03/023433 A1.
The NMR probe head of an NMR spectrometer is located, together with a measuring device, in the bore of a magnet cryostat. This magnet cryostat contains a superconducting coil which generates the magnetic field required for NMR measurements. The NMR probe head as well as the magnet cryostat must be kept at very low temperatures during operation. The thermal loss caused by thermal conduction and thermal radiation is therefore a problem.
For this reason, a refrigerator is conventionally provided for cooling the NMR probe head. Heat exchangers and a transfer line from the refrigerator to the NMR probe head transport the cooling power generated by the refrigerator. The NMR probe head is supplied with coolant via pumps or compressors and the transfer lines. The cooled components of the probe head are usually at temperatures of 10 to 60 Kelvin. A Gifford-MacMahon cooler (GM) or a pulse tube cooler (PT) is e.g. used as refrigerator.
The magnet cryostat of an NMR spectrometer comprises a helium tank which contains the superconducting magnet and liquid helium (LHe, 4.2 K), one or more radiation shields surrounding the helium tank, an outer vacuum container which is subsequently referred to as the outer shell, and one or more neck tubes which connect the helium tank to the outer shell. The radiation shields may also be containers which are filled with liquid nitrogen (77.3 K) to reduce the heat input into the helium tank. Helium and nitrogen are evaporated by the heat input into the helium tank and on the radiation shield due to radiation and thermal conduction through the neck tubes and further suspension means. To prevent evaporation of expensive helium and nitrogen, refrigerators (PT or GM coolers) are also used to cool the magnet cryostats.
In most cases, a cold finger is installed directly in the magnet cryostat. The cold finger is thereby connected to one or more shields in the cryostat and/or condenses evaporated helium (GHe) in the helium tank. This method is more efficient due to direct cooling compared to cooling using an external refrigerator and transport of the coolant via a transfer line. Such an arrangement with direct cooling is described in U.S. Pat. No. 6,389,821. In this method, more helium is condensed than evaporated. For this reason, part of the cooling power must be compensated for by an electric heating means. Part of the cooling power is thereby wasted.
WO 03/023433 A1 proposes use of a refrigerator cold finger which is installed in the magnet cryostat not only for cooling the cryostat but also for simultaneously cooling the NMR probe head. A large part of the transfer lines thereby extends within the cryostat which bears the risk that the occasionally required cleaning of the soiled NMR probe head could result in heat input into the magnet coil system and quenching of the magnet coil.
One would like to cool the NMR probe head and the magnet cryostat in a manner which is as simple and efficient as possible using a maximum amount of the cooling power produced by the refrigerator. This means that e.g. a thermal load at 60 Kelvin should not be cooled by a cooling source at 10K, since the efficiency would be very poor. Two-stage cryocoolers are therefore particularly suited for cooling elements at different temperatures, since cooling power can be tapped at two different temperature levels. The two temperature levels provided by the cryocooler are sufficient for cooling a helium tank and a radiation shield. Cooling of the NMR probe head, however, requires two additional temperature levels to cool the pre-amplifier and the resonator. The method described in WO 03/023433 A1 cannot optimize cooling, since it is not possible to use temperatures between the two temperature levels of the pulse tube cooler. Optimum cooling operation is therefore generally not possible with such devices. In the conventional devices, a considerable part of the input power of the cooler, approximately 4-8 kW, is still lost.
It is therefore the underlying purpose of the invention to propose an NMR arrangement wherein the probe head and magnet cryostat are cooled by a common refrigerator, permitting optimum utilization of the cooling resources of the refrigerator.